1. Field of the Invention
This invention relates to a server, a content providing apparatus and a content receiving apparatus that are suitable for application to the case of distributing content data such as video programs through a network such as the Internet, a content providing method and a content receiving method applied to these apparatuses, and a program for executing these methods.
2. Description of the Related Art
Conventionally, in a streaming service to distribute a relatively large volume of content data such as video programs through the Internet, one server for providing contents is installed or servers for providing contents are installed at one place, and many clients are connected to the server in a centralized manner.
Specifically, for example, as shown in FIG. 21, one server 1 is prepared in a network and plural CPEs (customer premise equipments) 2a to 2n are prepared, which are devices on the side of clients who receive service, and each of the CPEs 2a to 2n can receive distribution of content data prepared in the server 1. In this example, 14 CPEs 2a to 2n are prepared and seven CPEs 2a to 2g of them are now receiving distribution of content data from the server 1 while the other CPEs are on standby. In this example, only 14 CPEs are used for simplifying the explanation. In many cases of actual distribution of content data, more clients may access the server simultaneously.
Patent Reference 1 discloses distribution of content data or the like through a communication network such as the Internet.
Patent Reference 1: JP-A-2003-196491
Since clients access the server simultaneously, the server bears the burden of calculation and network corresponding to the total number of clients connected thereto. Generally, immediately after the content distribution service is started, many clients make requests simultaneously, and if the server does not have sufficient capability, trouble of server down occurs. If the server has sufficient capability to prevent such occurrence, the cost necessarily increases accordingly. Since the number of requests decreases with the lapse of time from the start of the service, the prepared server capability then turns out to be excessive investment. On the other hand, there also is a possibility that if the service becomes very popular, reinforcement of the server cannot catch up with unexpected increase in the number of requests for the service and server down occurs, losing opportunities of business. In such a content distribution system, for example, the number of expected simultaneous accesses to very popular contents may exceed one million.
In the example of FIG. 21, the seven CPEs 2a to 2g are receiving the service. In this case, the server 1 bears a load of arithmetic operation for the seven CPEs (often the same load of arithmetic operation as on the individual CPES) and a network load for the seven CPEs. Therefore, a device of higher cost than that of the device on the client side is demanded as the server. In the example of FIG. 21, in the worst cast, capital investment equivalent to the seven CPEs is necessary. Moreover, though the server must have capability to provide service to a statistically calculated maximum number of CPEs (in this case, for example, it is assumed that the maximum number of CPEs that simultaneously access the server is seven), the maximum number is not always reached at the time of ordinary service. If the server provides service to approximately two CPEs on average, the capability to provide service to the other five CPEs is considered to be excessive investment.
Also, in such content distribution systems, security is often weak. In many cases, in order to reduce the server load and server cost, encryption need not be performed. Therefore, there is a high risk of contents being stolen, and valuable contents are hardly distributed through networks. Even if encryption is performed, it is difficult to encrypt contents for each traffic. Generally, encryption in data distribution is applied only to a small volume of data (text data or the like). AS for large streams, data that have already been encrypted are distributed in order to reduce the cost. In this case, however, many CPEs commonly use the same key. Moreover, the contents are encrypted with the same key for a long period and the same key is sent many times. Therefore, it gives a clue to key analysis and the strength of encryption is low.